1991 Grand Marquis trouble shooting
 

Ok i have a 1991 Grand Marquis GS 4.9L (5.0) 302 windsor v-8 62k on engine,trans,body.

I'm a fledging mechanic that is going to schooling for mechanics and my job currently is a tire buster at a local shop. I have 4 years experience into mechanics and after owning this car for a few months i would like to tackle the problems i have been having with it and need the input of other MGM owners. If you have any answers or even ideas they would be greatly appreciated;)
Current work: Valve cover gaskets, Intake gasket, Throttle Postion Sensor(TPS), EGR valve, PCV valve, spark plugs, secondary wires, cap, rotor, fender mounted starter solenoid, fuel filter.

1.) The transmission will not shift into overdrive, when it comes time to shift into OD the car just revs up when throttle is applied as if it where in neutral. The problem is fixed when i drop it into 3rd gear but im wasting alot of MPG by not being able to shift into OD. Heard a rumor that it is a modulator valve or whatever the part is that is run off of manifold vaccum?

2.) The engine seems to idle at erratic levels at times and will even self accelerate when driving during this issue. No clue to the problem of this after i replaced the TPS.

3.) Coolant seems to just evaporate within a month. No leaking or burning, just dissapears after time. Not a big concern as i can add coolant/water easily and free.

4.) Gas mileage seems to be extremely bad, if i have a 1/4 tank than i can only drive for about 30 mins(regardless of city/HW). Full tank only lasts 2 hours or less!! Someone told me that they had a 91 MGM and the fuel sending unit used to read the gas the wrong way, anyone else with this problem?

Those are the main problems i'm having with this car, other non-performance problems are: No windows work, clear coat is chipped and fadded, pre-mature rust failure(body is extremely rusty even after being ziebart.ed, air bag light is flashing, passenger side floor in the rear gets soaked when it rains but there is no holes in floorpan??

your transmission problem is probably due to the bushing going bad that controls a cable connected to the transmission that adjusts pressure... that bushing going bad means you probably lost your overdrive gear and transmission needs rebuilt... the bushing holds a cable that is supposed to connect to the throttle... when the gas pedal is pushed, the transmission cable pulls forward away from firewall compared to being pulled toward the firewall like the accelerator cable... if you want... go to www.GrandMarq.NET and look up tv bushing.. that site also has lots to do with the 5.0 powered Crown Victoria, Grand Marquis and Town Cars

It sounds like your almost talking about the kickdown cable? it kicks down for passing gear when it should.

it is not passing gear... that cable is what controls pressure to transmission and if it has been disconected before and driven the transmission smoked the OD gear... somebody might have noticed it and fixed it but was too late. with the cable undone, the transmission runs at low pressure and doesnt work the hydraulics inside the transmission to cause enough pressure to make everything connect right and caused the clutches in the trans to slip


the idle issue could be the idle air control valve gummed up and making it idle funky..

Thanks for the reply i will definatly check that out on the transmission, and do you think replacing this part would make it shift better with the 3 gears it has left?

Also, about the idle air control valve. I have replaced it with a new one awhile back and it made it run better for maybe a week. Is it possible that the passages that the IAC use are gummed up or carbon coated? Because the part itself is brand new.